U.S. Pat. 5.473.887 discloses use of a NOx-storage catalytic converter for storing nitrogen oxides which are emitted by an internal combustion engine during a lean operation (lean air/fuel mixture, lambda>1).
U.S. Pat. 3,969.932 discloses applying the output signals of exhaust-gas probes for evaluating a 3-way catalytic converter in the context of an on-board diagnosis. The exhaust-gas probes are oxygen sensitive and are mounted forward and rearward of the catalytic converter. The known method is based on the oxygen storage capability of an operable 3-way catalytic converter. In this context, U.S. Pat. 3.969,932 discloses a change of the air/fuel mixture composition from lambda=0.95 (rich, fuel-rich mixture; oxygen deficiency) to lambda=1.05 (lean, low-fuel mixture, oxygen excess). The exhaust-gas sensor mounted forward of the catalytic converter reacts to a change of the air/fuel mixture composition virtually without delay. The exhaust-gas sensor mounted downstream of the catalytic converter, however, reacts only after a time span which is dependent upon the oxygen storing capability of the catalytic converter. The reason for this fact is that the oxygen storage locations of the catalytic converter are at first not occupied because of the oxygen deficiency present in the exhaust gas which is present at lambda=0.95. After the switchover to lean operation (oxygen excess) forward of the catalytic converter, the oxygen storage locations are successively occupied. For this reason, there continues to be at first an oxygen deficiency rearward of the catalytic converter after the change of the mixture composition. Only when the oxygen storage locations are occupied, an oxygen excess slowly builds up rearward of the catalytic converter which leads to a change of the output signal of the rearward exhaust-gas sensor. The time delay, that is, the phase shift between the reactions of, the two exhaust-gas sensors, can be used to evaluate the oxygen storage capability for the diagnosis of the catalytic converter.
Internal combustion engines having gasoline direct injection afford the advantage of reduced carbon dioxide (CO2) emissions. These internal combustion engines are operated mostly with a lean air/fuel mixture (lambda>1). For this reason, these engines are provided with a nitrogen oxide (NOx) storage catalytic converter which stores the NOx emissions arising during the lean mixture phase. Gasoline-direct injecting internal combustion engines are also operated at lambda=1 (homogeneous operation). For this reason, the NOx storage catalytic converters have, as a rule, also a storage capability for oxygen. For storing oxygen, a conventional 3-way catalytic converter can, for example, be used.
The storage capability of a catalytic converter with respect to nitrogen oxides and oxygen is limited. For this reason, the catalytic converter must be regenerated from time to time. During the discharge phase, a reducing agent is added to the exhaust gas via which the stored nitrogen oxides are reduced to oxygen and nitrogen. The reducing agent is, for example, configured as a hydrocarbon (HC) and carbon monoxide (CO) which can be inputted into the exhaust gas ahead of the catalytic converter via a rich mixture adjustment. Alternatively, urea can be added to the exhaust gas as a reducing agent. Here, for reducing the hydrogen oxide to oxygen and nitrogen, ammonia from the urea is used. The ammonia can be obtained from a urea solution by hydrolysis.
The time points for the start and end of the storage phase are important for the emissions discharged into the ambient rearward of the catalytic converter. During a lean operation of the internal combustion engine, the NOx storage catalytic converter is filled with nitrogen oxide and the 3-way catalytic converter is filled with oxygen. The start of the storage phase is determined via an NOx storing model. The NOx storing model models the nitrogen oxide quantity introduced to the NOx storage catalytic converter and so models the NOx fill level of this catalytic converter. If the modeled variable exceeds a pregivable threshold, then a discharge phase is initiated.
U.S. Pat. 6,216,451 discloses ending the discharge phase when an output signal of an exhaust-gas sensor, which is mounted rearward of the catalytic converter, exceeds a pregivable threshold value. Because of manufacturing tolerances, deterioration and temperature fluctuations, fluctuations of the output signal of the rearward exhaust-gas sensor can, however, occur which can lead to an early or a late ending of the discharge phase. The consequences are inadequate utilization of the maximum storage capability of the catalytic converter because of a discharge phase which ends too early or exhaust-gas emissions which are too high, especially an emission of the reducing agent in a discharge phase which ends too late.